Call of Duty is fun. I really enjoy it. It knows what it is, it's no tactical shooter and it's not realistic. But it just oozes atmosphere and has an uncanny ability to place you in the middle of your favorite movies without you being aware of it. Plus Modern Warfare was just a masterpiece of cinematic storytelling and pacing in a video game. Its story wasn't omg spectacular but it really took advantage of the medium to provide a unique experience. The nuclear detonation and the SAS' last stand on the bridge are just spectacular sequences.
World at War gets a lot of flak because it's made by Treyarch but I really enjoy it. It's a more traditional Call of Duty game than Modern Warfare (and not just because it's set in WW2) but it's also the best of the traditional Call of Duty games. I love the Russian campaign. "Citizens of Berlin, abandon all hope!"

Man don't even get me started on Rainbow Six. That used to be one of the best FPS franchises out there but they ran it into the ground. Call me old school but I miss planning your attacks, coordinating with other teams, all that jazz. The current Rainbows have all these massive shootouts and action set pieces that just have no place at all, in the originals if you were involved in a big shootout it was because something went horribly wrong. And does anyone else miss taking casualties? You lose a lot when you make the teammates invulnerable. I still remember one mission in Rogue Spear that went like clockwork, terrorists eliminated with the uttermost efficiency, but at the last minute as we were saving the hostages one of the "tangos" got off a shotgun burst that killed one of the teamleaders and he stayed dead the rest of the game. Rainbow Six Vegas just SUCKS in comparison!